As a model for understanding RNA-protein interactions we are studying the structure of the Rev-Response Element RNA in complex with peptides derived from thRev protein. Rev is one of two small basic regulatory protein in HIV. Binding of Rev to the Rev-response element RNA (RRE) results in a change in the ratio of spliced and unspliced viral mRNAs that appear in the cytoplasm. Formation of this interaction is a key point in the replication cycle of HIV. The structure determination of this RNA-peptide complex is nearly complete. Nine different samples have been prepared with various isotopic labeling combinations in the RNA and peptides. Approximately 100 torsion restraints, 400 RNA NOE restraints, 350 peptide NOE restraints, and 60 RNA-peptide NOE restraints have been collected. Preliminary structure calculations reveal that an alpha-helical peptide is inserted into the major groove of the RNA, which adopts an unusual structure due to the presence to two purine-purine base pairs.